docs: Add pull_request event

Author: peaceiris

README.md

@@ -66,6 +66,7 @@ on:
   push:
     branches:
       - main
+  pull_request:
 
 jobs:
   deploy:
@@ -83,6 +84,7 @@ jobs:
 
       - name: Deploy
         uses: peaceiris/actions-gh-pages@v3
+        if: github.ref == 'refs/heads/main'
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: ./book